Present antenna arrays used for transmitting and receiving RF (Radio Frequency) signals in mobile communication systems are normally dedicated to a single frequency band or sometimes two or more frequency bands. Single frequency band antennas have been used for a long time and normally include a number of antenna elements arranged in a vertical row. A second row of antenna elements needs to be added beside the first row if the operator in a network wants to add another frequency band using single frequency band antennas. However, this requires enough space to implement and the arrangement may also be sensitive to interference between the RF signals in the different frequency bands.
These drawbacks have been partially resolved by prior art arrangements 10 which are schematically shown in FIGS. 1A and 1B.
In FIG. 1A two types of antenna elements 11, 12 have been arranged alternatively in a column. A first antenna element 11 is a dual band antenna element which operates in two different frequency bands FB1 and FB2, a second antenna element 12 is an antenna element which operates in only one frequency band FB1. A drawback with this prior art embodiment is that the frequency bands FB1 and FB2 will couple to each other due to the closeness of the parts making up the antenna element 11.
Therefore, this kind of configuration is only suitable when the frequency bands have a big separation, for example if FB2 is approximately twice the frequency as FB1. If the frequency bands are too close, filters with high Q values, for example cavity filters which consume space and are relatively expensive and heavy, must be used very close to the antenna elements.
The prior art arrangement shown in FIG. 1B, as disclosed in U.S. Pat. No. 6,211,841 (Nortel), is formed by an array including first antenna elements, 11a, which are positioned in two parallel columns 13a, 14a and operate in a first, lower frequency band, and second antenna elements 12a, which are alternately located in two adjacent columns 13a, 15a and operate in a second, higher frequency band. One of these adjacent columns (13a) is the same as one of the columns accommodating the first antenna elements 11a, whereas the other column 15a is located between the columns 13a, 14a. By locating the antenna elements 11a, 12a in parallel, spaced apart columns side by side, it has been made possible to achieve the desired low coupling even between frequency bands which are relatively close to each other, namely up to a quotient of about ⅔.
In U.S. Pat. No. 6,844,863 B2 (Andrew Corporation), an arrangement with interleaved arrays of antenna elements is disclosed. Here, the various arrays deliberately couple to each other in a common frequency band.
Accordingly there is a need for a new antenna arrangement that will operate in two or more frequency bands with a reduced coupling between the frequency bands without using filters close to the elements or, if filters are needed, using filters with low Q values, such as micro strip or strip line filters, which are small in size and relatively cheap to implement.